Method of providing contacts on semiconductive ceramic bodies of n-type oxidic material and contacts produced thereby



" FIGJ OF PfiOVIDIN ONTACTS RAMIC BODIES N-TY OXIDIC TER y 4, 1965 R M. A. DE PROO ETAL 3,182,118

MET D SEMI-CONDUCTIVE IAL AND CONTACTS PRO ED Y Filed April 17, l v

SENHC/ONDUCTOI/ ,NVENTOR.

AG NT United States Patent ce 3,182,118 METHOD OF PROVIDING CONTACTS 0N SEMI- CONDUCTIVE CERAMHC BODIES 0F n-TYPE OXIDIC MATERIAL AND CQNTACTS PRO- DUCED THEREBY Roger Marie Albert de Proost and Christiaan Alfons Maria Klaassen, Brussels, and Jean Michel Baudry Ghislain Yperman, Sint-Steven-Woluwe, Belgium, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Apr. 17, 1963, Ser. No. 273,708 Claims priority, application Netherlands, Apr. 19, 1962, 277,479 8 Claims. (Cl. 174-94) The invention relates to the provision of contacts on semi-conductive ceramic bodies which consist of an n-type oxidic material. As is known, such materials are obtainable by partial reduction of the stoichiometric compounds, for example, titanium dioxide, part of the metal ions of the compounds assuming a lower valency. As is also known, this change in valency not only can be produced by reduction, but also by the incorporation of ions of different valency. This is the case, for example, with sintered products of ferric oxide F6 0, containing about 1% of titanium dioxide, (see U.S. patent specification No. 2,735,824). Other examples of the said bodies and consisting of e.g. sintered barium titanate containing 0.3% of antimony oxide are described in British patent specification 714,965.

The said semi-conductive materials generally have a negative temperature coefiicient of resistance. In some cases, however, the temperature coefiicient assumes a positive value. As is known, this is the case with compositions consisting mainly of alkaline earth titanates, for example, barium titanate containing small additions of oxides of rare earth metals, antimony, tungsten or the like. All the n-conductive materials concerned are susceptible to oxidation, which may occur in the provision of contacts if this includes a heat treatment in air. As a result, a barrier layer of oxidized semi-conductor material is produced between the semi-conductor material and the contact so that the contact resistance becomes dependent on the direction of current flow and on the magnitude of the voltage. In a resistance body provided with two of such contacts, the resistance may assume a very high value.

It is known that in resistors consisting of an oxidizable sintered resistance material which is conductive due to the presence of an excess of cations, the said difficulties can be obviated by joining the resistance material and the current supply contact with the aid of a sintered or molten metallic material which contains not only the metal, an excess of which is present in the resistance material in the form of cations, but also the metal of which the contact member consists.

This method of providing contacts has a limitation in that, with a given composition of the semi-conductor body and of the contact member, the connection sometimes has to be made with the aid of alloys which have a high melting point and/ or poorly wet the semi-conductor body, so that a contact having a satisfactory adherence is not always obtainable.

A frequently used method of providing firmly adhering contacts on ceramic materials consists in that a coating is produced by firing silver to the ceramic materials with the use of a suspension containing silver and/ or a silver compound, for example silver oxide or silver acetate. The thickness of the silver layers is for example 10 microns; the firing occurs by heating in air at about 800 C. This manner of contacting, which is carried out with heating in air, is simple and also has the advantage that 3,182,118 Patented May 4, 1965 contact members of a variety of shapes and compositions may be secured to the silver layer by soldering.

For the provision of contacts on n-type ceramic bodies, however, this method cannot be used because due to oxidation of the semi-conductor, barrier layers are formed between the bodies and the silver layers produced by firing.

The present invention is based on the recognition that this oxidation which occurs in the provision of silver layers can be counteracted by the presence of a highly reducing metal in the junction layer so that the occurrence of barrier layers is effectively prevented.

If this result is sought by applying a reducing metal to the contact area before the silver layer is produced by firing, a contact is obtained which is unsatisfactory mechanically and/ or electrically.

It has, however, been found that, if zinc and/ or tin are used for the above-mentioned purpose as the reducing metal or metals, the treatment with such metal or metals can be carried out after the provision of the silver layer, because even at a comparatively low temperature these metals diffuse through the silver layer at a rate which is sufficient in practice to counteract oxidation.

According to the invention, a contact is provided on a semi-conductive ceramic body consisting of an n-type oxidic material by coating the body by firing silver thereto from a suspension of silver and/ or a silver compound, after which at a temperature exceeding C. Zinc and/ or tin is or are difiused through the silver layer into the surface layer of the semi-conductor body. The temperatures used in this process are preferably maintained below 400 C. because otherwise excessive oxidation of the reducing metal may take place.

The reducing metal may be provided on the layer of silver electrolytically, by spraying, by deposition from the vapour phase or with the aid of a suspension, after which the above-mentioned heat treatment is carried out. Current-supply members can then be provided by soldering.

It has been found that the provision and diffusion of the zinc and/or tin and the soldering may be carried out in a single operation by using a solder containing these metals. Since the diffusion of tin through the silver layer proceeds at a much lower rate than the diiiusion of zinc, in this embodiment of the invention the layer has to be afterheated for a comparatively long period of time if a solder containing tin but little or no zinc is used.

Solder having a melting point not exceeding about 350 C. is preferably used.

As is known, in the process of soldering, in order to prevent injury to the silver layers produced by firing, it is of advantage to use a solder containing silver.

It has been found that solders containing lead generally give results which are appreciably inferior to those obtained with alloys on the basis of cadmium and/ or tin.

The contacts in accordance with the invention are very satisfactory mechanically and completely free from barrier-layer phenomena. They also proved to remain completely stable even after prolonged use in air. This may be due to the presence of an excess of zinc and/ or tin in the junction layer which prevents the occurrence of oxidation and hence the formation of barrier layers.

By way of example, a plate of sintered barium titanate containing 0.3% of antimony oxide having a diameter of 8 mms. and a thickness of 2 mms. is provided on both sides with contact layers produced by firing silver to it in known manner. The resistance measured between the contacts exceeds 10 ohms. The silver layers are then coated with zinc by spraying. After a heat treatment at 300 C. for 10 minutes, the resistance is found to be reduced to about 10 ohms. Current-supply wires can now be soldered to the resulting contacts, for example, with a normal lead-tin solder.

Similar results were achieved in providing contacts on resistance bodies consisting, for example, of sintered ferric oxide (Fe O containing 1% of titanium dioxide, sintered cobalt ferrite and similar n-type resistance materials.

When a solder is used which consists of an alloy of, for example 81.5% of cadmium and 18.5% of zinc, the application of zinc to the silver layer, the diffusion of the zinc through this layer and, if required, the soldering of current-supply wires to the resulting contacts can be carried out in a single operation at a soldering temperature of 300 C. In this case, there is sutficient diffusion of zinc through the silver layer so that there is no need for further heating.

When a solder having a low Zinc content 01' a solder containing only tin for counteracting the oxidation produced by the silvering process is used, the heat treatment 7 must be prolonged toachieve the desired result.

If, for example, a solder is used which consists of a eutectic alloy of tin and cadmium (67% Sn, 33/ Cd) with an addition of 2% of zinc, heating to 200 C. for half an hour is required. After soldering with a solder which consists, for example, of an alloy containing 92% of tin and 8% of zinc to which 5% of silver is added, at a temperature of 250 C., or of an alloy containing 80% of tin and 20% of silver at 300 C., heating to 220 C. has to be continued for half an hour. Whether or not heating after soldering has to be applied will depend on e.g. the content of silver in the solder, the composition of the solder, the temperature at which the soldering is carried out and the time of soldering. In practice a man skilled in the art will be able to determine when it will be sufficient to apply soldering without additional heating with the invention according to the present application being disclosed to him.

Examples of parts of a semi-conductor component according to the present invention are shown in FIGS. 1 and 2, which are cross-sectional views thereof.

FIG. 1 refers to the example as described above in which the n-type semi-conductor is a plate of sintered barium titanate containing 0.3% of antimony oxide, the plate, having a diameter of 8 mm. and a thickness of 2 mm., has been provided with a layer of fired silver of about 10 microns, obtained by spraying a suspension consisting of 55% by Weight of silver, 5% by weight of nitro-cellulose and 40% by weight of ethyl-acetate on the plate and heating in air at about 800 C. during 10 minutes. The silver layer has been provided with a layer of about 5 microns of Zinc (Zn) by spraying, whereafter by a heat treatment at 300 C. during 10' minutes part of the zinc has been diffused through the layer of fired silver into the body of the n-type semi-conductor.

FIG. 2 represents part of a semi-conductor component according to the present invention in which the body of the n-type semi-conductor has been provided with a layer of about 8 microns thickness of fired silver in a manner as described above. On the silver layer a lead is soldered using as a solder an alloy consisting of 81.5% by Weight of cadmium and 13.5% by weight of zinc a soldering temperature of about 300 C. having been applied.

What is claimed is:

1. A method of making a satisfactory ohmic contact to a semi-conductive ceramic body of n-type conductivity oxidic material, comprising the steps of coating a portion of the body with a layer containing silver, firing the silver layer, and thereafter diffusing an element selected from the group consisting of zinc and tin at a temperature exceeding C. through the fired silver layer into the semi-conductive body to prevent oxidation at the interface and maintain the contact resistance low.

2. A method as set forth in claim 1 wherein the diffusion step is carried out by depositing a layer of said element on the fired silver layer, and then heating at a temperature between 150 C. and 400 C. to effect the diffusion.

3. A method as set forth in claim 1 wherein a connecting lead is thereafter soldered to the silver layer.

4. A method of making a satisfactory ohmic contact to a semi-conductive ceramic body of n-type conductivity oxidic material, comprising the steps of coating a portion of the body with a layer containing silver, firing the silver layer, and thereafter solderinga connecting lead to the fired silver layer using a zinc-containing solder constituted predominantly of at least one element selected from the group consisting of tin and cadmium at a temperature exceeding 150 C. to difiuse the zinc through the fired silver layer into the semi-conductive body to prevent oxidation at the interface and maintain the contact resistance low.

5. A method as set forth in claim 4 wherein the solder has a melting point below about 350 C.

6. A semi-conductor component comprising a semi-conductive ceramic body of n-type conductivity oxidic material, a fired silver layer on a surface portion of said body, an electrical connection to the silver layer, and an element selected from the group consisting of zinc and tin being present at the interface of the silver and the body and having been provided thereat by diffusion through the silver layer to lower the contact resistance.

7. A semi-conductor component comprising a semi-conductive ceramic body of n-type conductivity oxidic material, a fired silver layer on a surface portion of said body, and an electrical connection soldered with a soldering material to the silver layer, said soldering material containing an element selected from the group consisting of zinc and tin, part of which during the soldering diffuses through the silver layer to lower the contact resistance.

8. A component as set forth in claim 7 wherein the soldering material is predominantly of an element selected from the group consisting of tin and cadmium and contains a small amount of zinc.

References Cited by the Examiner UNITED STATES PATENTS 2,371,613 3/45 Fair 29473 X 3,010,188 11/61 Bol et a1 29-473 X FOREIGN PATENTS 564,587 10/38 Canada.

E. JAMES SAX, Primary Examiner.

JOHN P. WILDMAN, Examiner, 

6. A SEMI-CONDUCTOR COMPONENT COMPRISING A SEMI-CONDUCTIVE CERAMIC BOSY OF N-TYPE CONDUCTIVITY OXIDIC MATERIAL, A FIRED SILVER LAYER OF A SURFACE PORTION OF SAID BODY, AN ELECTRICAL CONNECTION TO THE SILVER LAYER, AND AN ELEMENT SELECTED FROM THE GROUP CONSSITING OF ZINC AND TIN BEING PRESENT AT THE INTERFACE OF THE SILVER AND THE BODY AND HAVING BEEN PROVIDED THEREAT BY DIFFUSION THROUGH THE SILVER LAYER TO LOWER THE CONTACT RESISTANCE. 